1. Field of the Invention
The present invention relates to a technology of performing a processing to reduce a bit number to a frequency signal made of a digital signal outputted from a DDS (Direct Digital Synthesizer) or the like.
2. Description of the Related Art
In a device outputting an analog frequency signal based on a digital signal (bit data) generated in a signal generation section, such as a DDS and a digital audio player, sometimes a bit number of a signal processable in a digital/analog (D/A) conversion circuit is smaller than a bit number of a digital signal generated in the device for example. In such a case, it is necessary to perform a processing (hereinafter, referred to as a reduction processing) to reduce the bit number to the digital signal having been generated by the signal generation section.
In reduction of a bit number as above, multiplying an n-bit digital signal by “2−m (m<n)” for example enables the bit number to be reduced by m bits.
Here, the present inventor performs a frequency analysis of digital signals before and after performing a reduction processing of a bit number in a series of analyses to pursue improvement of a performance of a DDS, and confirms that an unnecessary frequency component (spurious) which has not been found before the reduction processing occurs in the digital signal after the reduction processing. Such a spurious is maintained also after an analog conversion and causes deterioration of a characteristic of a device.
Here, though there is described in Patent Document 1 a technology to suppress occurrence of a spurious of a frequency signal by adding noise data to make an ensemble mean zero to a cumulative addition section exit of a DDS at a random timing, no attention is paid to a spurious which occurs in reducing a bit number of a digital signal having been generated in DDS. Further, if a method described in Patent Document 1 is adopted and waveform data of the DDS is outputted to an A/D conversion section, it is anticipated that a floor noise or a phase noise increases.
[Patent Document]
    [Patent Document 1] Japanese Patent Application Laid-open No. 2000-252750: paragraph 0024 to paragraph 0029, FIG. 1